This invention relates generally to processes for the separation of noble gases and more specifically to processes for the separation of xenon from a gaseous mixture containing krypton.
There is increasing interest in new methods for noble gas separation for reasons of both cost and safety. The prevalent separation method for xenon and krypton is cryogenic distillation generally as a byproduct from the manufacture of liquid air. This process, while well developed and reliable, is relatively energy intensive. Another source for these noble gases may become available in the future, as the products from nuclear fission, found as offgases from either nuclear reactors or reprocessing plants. A number of novel rare gas separation methods have been recently developed to both take advantage of this new source and also reduce or eliminate the radioactive hazards found with these offgases.
The separation costs arise primarily from the thermodynamically inefficient production of low temperatures required for separation and near liquid nitrogen temperatures (-196.degree. C.). While the amount of rare gases separated constitute only a very small fraction of the volume processed by a commercial liquid air plant, noble gas purity requirements involve extensive amounts of distillation. Elimination of this feature, as found in the room temperature process described here, would reduce processing costs. Furthermore, a safety hazard is found when the cryogenic process is used with the gaseous products from spent nuclear fuel. Radiolysis of the air and water always present produces ozone, which becomes explosive when condensed as a liquid at temperatures below about -120.degree. C. The incentives for developing a noncryogenic separation technique are thus clear.
Of the isotopes of Xe and Kr produced by nuclear fission, only one, .sup.85 Kr, has a significant half-life (of about 10 years). Furthermore, krypton constitutes only 6.5% of the total rare gases, so that a simple elemental separation process is sufficient to produce xenon free of any radioactive isotopes. Further photochemical enrichment of this isotope is possible but not necessary for most applications.
The discovery of the first noble gas compounds was soon followed by the first reports that some of these compounds could be synthesized photochemically. Xenon difluoride (XeF.sub.2), xenon tetrafluoride (XeF.sub.4) and xenon hexafluoride (XeF.sub.6) can be made thermally from Xe and F.sub.2 ; however, temperatures of about 250.degree. C. are necessary for establishment of equilibrium on a reasonable time scale. It is relatively straightforward to make XeF.sub.2 photochemically, but photochemical production of KrF.sub.2 requires quite severe conditions such as in the cryogenic liquid or solid states. This difference in stabilities between the two difluorides is the basis for the separation technique presented here.
Xenon difluoride is produced when a mixture of xenon and fluorine is irradiated in the ultraviolet (UV), into the absorption band of the fluorine molecule. This band is relatively weak (.epsilon. of about 3.0 1.mol.sup.-1 cm.sup.-1) and broad, peaking at 285 nm, and photolysis in this region results in the production of fluorine atoms. Any photolytic source can be used as long as the wavelength is appropriate, from about 250 to 350 nm. It is not necessary that the mechanism resulting in XeF.sub.2 production be totally understood. Indeed, some of the details have yet to be fully explained. The significant feature is that a photochemical process involving the rare gases can be made selective for xenon, where the products are molecular and can be readily separated from reagents.